1. Field of the Invention
The present invention relates to a thin film write head and more particularly to a thin film write head in which a second pole tip with narrow track width can be patterned and plated simultaneously with a second pole piece.
2. Description of the Related Art
A thin film write head includes first and second pole pieces which are magnetically connected in a pole tip region and at a back gap. In the pole tip region the first and second pole pieces provide first and second pole tips which are separated by a thin insulative gap layer. The pole tip region is defined by a head surface and a zero throat height between the head surface and the back gap. A yoke or body portion of the head lies between the zero throat height and the back gap. In the body portion of the head there are located one or more layers of pancake coils and a plurality of insulation layers. The pancake coils couple flux into the pole pieces and/or receive flux therefrom.
Each of the insulation layers has an apex near the pole tip region where the insulation layer commences. Each apex is located at or between the zero throat height and the back gap. In the prior art the apex of a first insulation layer above the first pole piece is typically located at and defines the zero throat height. Each insulation layer has a steep sloping surface from its apex to its highest level above the first pole piece. This slope is caused by a process step in which the insulation layer is heated. The heating process drives out solvents causing the insulation layer to shrink and slope from the apex to the highest level.
The second pole piece has a flare point at which it widens beyond the width of the second pole tip to form a large main body portion. This flare point is located between the zero throat height and the back gap. If the flare point is set too far back towards the back gap flux may leak from the pole piece due to the extra length of narrow material through which the flux must be transmitted. However, setting the flare point too close to the zero throat height may prevent simultaneous fabrication of the second pole piece and a high resolution narrow track width second pole tip as will be explained in more detail hereinafter.
The thickness of the gap layer between the second pole tip and the first pole tip, and the configuration of the second pole tip are the most crucial elements in a thin film write head. The thickness of the gap layer at the head surface determines the linear density of the head, namely how many bits per linear inch of a magnetic medium the head can write. The width of the second pole tip determines head track width, which establishes how many tracks per width of a magnetic medium in inches can be written by the head. The product of these two factors is areal density. With present day demands for storing and processing large amounts of data, such as in high definition television (HDTV), there is a strong felt need for a thin film write head which provides high areal density by way of a high resolution, narrow track width second pole tip.
A high resolution second pole tip can be made by an image transfer process followed by reactive ion etching. The image transfer process typically masks the top surface of a resist layer with a metal pattern which is unaffected by reactive ion etching. The area not covered by the mask is where the pole tip is to be plated, and this area is shaped by reactive ion etching. The steps of depositing the metal pattern and etching are very costly. The second pole tip can also be made by ion beam etching in which the second pole piece is bombarded with ions to form a second pole tip with a desired track width. This process is also very costly. In both of these methods the second pole tip is constructed individually and then the remainder of the second pole piece is stitched to the second pole by ordinary photolithography and plating.
The least costly process for making the second pole tip is to construct it with the same process steps which construct the second pole piece. These process steps employ a single photoresist layer which can be patterned for plating the entire second pole piece along with the second pole tip in a single operation. However, prior art methods of constructing the second pole piece and the second pole tip with the same process steps have not provided a high resolution second pole tip. When the second pole piece and the second pole tip are constructed simultaneously by ordinary photolithography a photoresist layer is spin coated onto the body portion and pole tip region of the head. The photoresist layer is located above a gap layer in the pole tip region and above a stack of insulation layers in a coil region. The insulation stack is typically 7 to 8 microns (.mu.m) above the gap layer and has a marked slope as the first insulation layer transitions to its apex at the zero throat height closely followed by the pancake coil and the insulation layers in the coil region. When the resist is spin coated onto a wafer it substantially planarizes across the body portion and the pole tip region, causing the resist in the pole tip region to be considerably thicker than the resist in the body portion of the head. The thickness of the resist in the body portion of the head is dictated by the desired thickness of the second pole piece. For example, if the second pole piece in the body portion is to be 4 .mu.m thick the photoresist layer would have to be approximately 4.5 .mu.m thick. With a typical insulation stack of about 8 .mu.m this results in the resist layer being about 11 .mu.m thick in the pole tip region. This thickness in the pole tip region plus the steep slope of the insulation layers near the pole tip region makes it very difficult to construct a narrow track width second pole tip with subsequent photolithography steps. In a viable manufacturing process for making high resolution thin film write heads the aspect ratio of the thickness of the photoresist layer with respect to the track width of the pole tip should be in the order of 4 to 1. Accordingly, the thickness of the photoresist should be no more than four times the desired track width of the second pole tip.
After the photoresist layer is deposited it is patterned by the exposure of light in one or more areas which are to be removed by a subsequent step of dissolving the exposed photoresist. Because of the thickness of the photoresist in the pole tip region the intensity of the light for patterning has to be high in order to penetrate the full depth of the photoresist. When the intensity of the light is high the narrow slits employed for patterning miniature features introduce deflective components in the light at the edges of the slits, which causes the light to fringe as it strikes the photoresist. This results in poor resolution. A more serious problem however is the reflection of light into the pole tip region from sloping insulation layers behind the zero throat level. In an aggravated situation assume that the flare point of the second pole tip is to be in the same plane with the zero throat height. The patterned photoresist layer commences its flare at the zero throat height and widens quickly toward the back gap to the full width of the second pole piece. This exposes a large expanse of the sloping portions of the insulation layers immediately behind the pole tip region. When light is exposed in these areas it is heavily reflected at an angle of incidence from the sloping portions of the insulation layers into the pole tip region where it is not wanted. The result is that the reflected light notches the photoresist layer in the pole tip region, substantially reducing the resolution of the second pole tip. Plating after this type of patterning results in a second pole tip which has irregularly shaped side walls and a poor size control of line width.
A solution to the reflection problem is to move the flare point further away from the zero throat height towards the backgap. If the flare point is pushed far enough back the reflected light will not reach the pole tip region. The light will simply be reflected into a narrow portion of the pole piece area behind the zero throat height where notching occurs without any substantial effect on the size control of the second pole tip. However, moving the flare point rearwardly extends the length of this narrow portion through which flux must be transferred from the large part of the second pole piece to the second pole tip, resulting in flux leakage from the narrow portion, which severely degrades the performance of the head.